1. Field of the Invention
The present invention generally relates to recording methods and apparatuses using an intermediate transfer medium and, more particularly, to a method and apparatus for forming images on the front side and back side of a sheet-like recording medium such as printing paper by forming images on an intermediate transfer medium by applying a liquid onto a layer of the intermediate transfer medium formed on an intermediate transfer member and transferring the thus-formed images to the front side and back side of the sheet-like recording medium.
2. Description of the Related Art
A recording method such as an inkjet recording method, in which droplets of a liquid containing a colorant are projected onto a recording medium in accordance with image signals, can be accomplished by an apparatus having a relatively simple, compact and light structure and a low noise. However, the inkjet recording method has a problem with respect to print quality in that the results of printing differ due to differences in the quality of printing papers such as synthetic paper, regular paper or inkjet paper. Particularly, when the printing is performed on regular paper with general use water soluble ink, there is a problem in that the resolution of a printed image is decreased due to running of ink on the printing paper or penetration of ink to the back side of the printing paper. Additionally, there is a problem in that the printed ink image may be smeared due to undried ink being touched when the printing paper having the ink image thereon is ejected from the inkjet apparatus. Further, since water soluble ink is normally used in the normal inkjet recording method, when the printing is performed on regular printing paper, printing cannot be performed on the back side of the printing paper after printing the front side of the printing paper due to running of ink on the printing paper or penetration of ink to the back side of the printing paper.
In order to eliminate the above-mentioned problems, intermediate transfer methods have been developed such as disclosed in U.S. Pat. No. 4,538,156 and No. 5,099,256. In the intermediate transfer methods, an ink image is formed on an intermediate transfer member according to the inkjet recording method, and, thereafter, the ink image is transferred onto a recording medium such as printing paper. According to the intermediate transfer method, the recording head can be located remote from the printing paper, which prevents clogging of the nozzles of the recording head due to adhesion of paper dust. However, the intermediate transfer method has problems in that, when droplets of ink are provided on the intermediate transfer member, the droplets may improperly spread or move on the intermediate transfer member. Additionally, if color ink is used, mixing of colors may occur which causes an image quality to deteriorate. Further, there are issues to be solved such as the transferring efficiency to a recording medium such as paper or the quality of the transferred image on the recording medium. For example, running of ink or penetration of ink to the back side of the printing paper is one of the issues to be solved.
In order to solve the above-mentioned problems, Japanese Laid-Open Patent Application No. 62-92849 discloses an improved intermediate transfer method in which the droplets of ink on the intermediate transfer member are dehydrated so as to transfer the condensed ink droplets to a recording medium such as paper. This method is advantageous in forming a clear image. However, the condensation of ink requires a considerable time interval and amount of heat energy, and also requires means for preventing scattering of water vapor from the ink droplets. Accordingly, this method cannot satisfy the demand for high-speed printing for both sides of a recording medium, the demand for energy saving and the demand for a further increase in the resolution of the image.
Japanese Laid-Open Patent Application No. 5-200999 discloses an intermediate transfer method in which droplets of ink are applied to a water absorbing layer on the intermediate transfer member so as to dehydrate the droplets, and the dehydrated droplets are transferred to printing paper. The water absorbing layer is made of a film or fibrous material which swells by absorbing water at a low temperature and releases the absorbed water due to a phase change at a high temperature. That is, in this method, water or solution in the droplets of ink is removed by the water absorbing layer, and the condensed ink is transferred onto the printing paper. Accordingly, this method is advantageous in forming a clear image. However, similar to the method disclosed in Japanese Laid-Open Patent Application No. 62-92849, the condensation of ink requires a considerable time interval and a considerable amount of heat energy, and also requires means for preventing scattering of water vapor from the ink droplets. Additionally, this method requires means for removing the water or solution from the water absorbing layer. Accordingly, this method cannot satisfy the demand for high speed printing for both sides of a recording medium, the demand for energy saving, the demand for a further increase in the resolution of the image and the demand for reducing the size and weight of the apparatus.
Japanese Laid-Open Patent Application No. 7-89067 discloses an inkjet recording technique in which droplets of ink are provided on a surface active agent applied on the intermediate transfer member so as to form an ink image on the surface active agent so that the thus-formed ink image is transferred onto a recording medium. Generally, the surface active agent has a function to reduce surface tension of a liquid or a solution. Thus, when a droplet of ink whose major component is water is provided onto the surface active agent by an inkjet recording method, the surface tension of the droplet of ink is reduced. The reduction in the surface tension results in the spreading of the droplet of ink. Accordingly, the diameter of the droplet of ink on the surface active agent is larger than the diameter of the droplet of ink projected by the inkjet nozzle. Thereby, it is difficult to achieve a high-resolution, sharp image by the technique using the surface active agent. Additionally, since the droplet is transferred onto the recording medium such as paper together with the surface active agent, the droplet of ink and the surface active agent are absorbed by the recording medium, which results in the running and penetration of ink. Thus, the technique using the surface active agent cannot be used for both-side printing.
Additionally, Japanese Laid-Open Patent Application No. 10-58664 discloses a method in which droplets of ink containing quarternary ammonium salt are provided onto an anionic ion exchanging resin layer on the intermediate transfer member so as to transfer the droplets of ink after the droplets are subjected to a chemical reaction during the conveyance of the droplets by the intermediate transfer member. This method is advantageous in improving the transfer efficiency. However, since the nature of the colorant contained in the ink is changed by the ion exchanging resin layer, the color of the ink may be slightly changed. Generally, the ion exchanging resin layer does not have a function to absorb water contained in a droplet of ink. Accordingly, the droplet of ink applied to the ion exchanging resin layer is subjected to a change in its nature, but concentration of ink in the droplet does not change. That is, the droplet of ink is transferred onto a recording medium without change in the concentration of ink. Thus, running of ink, color mixing or penetration of ink to back surface occurs in the transferred ink image. Additionally, when the ion exchanging resin layer is heated so as to restore the function of the ion exchanging resin layer, an ammonium gas is generated. Accordingly, the stench of the ammonium gas causes pollution of air. Further, there is a problem in that a maintenance operation must be frequently performed so as to restore the ion exchanging resin layer. Further, this method cannot satisfy the demand for high-speed printing for both sides of a recording medium, the demand for energy saving, the demand for a further increase in the resolution of the image and the demand for reducing the size and weight of the apparatus.
In order to solve the above-mentioned problems, Japanese Laid-Open Patent Application No. 11-188858 suggests an intermediate transfer method using a polymeric powder being applied onto an intermediate transfer member, the polymeric powder having a liquid absorbing characteristic. In this method, a layer of the polymeric powder is formed on an intermediate transfer member before a liquid is provided onto the intermediate transfer member so as to form an image. The polymeric powder dissolves in the liquid or swells by absorbing the liquid, and increases the viscosity of the liquid. Additionally, the layer of the polymeric powder layer can be easily removed from the intermediate transfer member. However, this patent document does not refer to a method for recording images on both sides of a sheet-like recording medium such as printing paper without deterioration of the image quality due to running or penetration of ink.